In recent years, miniaturization of electronic components have been advanced rapidly as the electronics technology has been developed. In the field of monolithic ceramic capacitor as well, demands for miniaturization and increases in capacity have intensified. Therefore, development of ceramic materials having high relative dielectric constants and reduction in thickness and multilayering of dielectric ceramic layers have been advanced.
For example, Patent Document 1 proposes a dielectric ceramic represented by a general formula: {Ba1-x-yCaxReyO}mTiO2+αMgO+βMnO (where Re is a rare earth element selected from the group consisting of Y, Gd, Tb, Dy, Ho, Er, and Yb, and α, β, m, x, and y satisfy 0.001≦α≦0.05, 0.001≦β≦0.025, 1.000<m≦1.035, 0.02≦x≦0.15, and 0.001≦y≦0.06, respectively).
Patent Document 1 discloses a monolithic ceramic capacitor including the above-described dielectric ceramic. The monolithic ceramic capacitor having a thickness of 2 μm per ceramic layer, the total number of effective dielectric ceramic layers of 5, a relative dielectric constant ∈r of 1,200 to 3,000, and a dielectric loss of 2.5% or less can be obtained.
The monolithic ceramic capacitor of Patent Document 1 takes advantage of a dielectric action of the ceramic itself. On the other hand, research and development on semiconductor ceramic capacitors based on a principle different from this have also been conducted intensively.
Among them, a SrTiO3 based grain boundary insulation type semiconductor ceramic is produced by firing (primary firing) a ceramic compact in a reducing atmosphere to convert the ceramic compact to a semiconductor, coating the ceramic compact with an oxidizing agent containing Bi2O3 or the like and, thereafter, conducting firing (secondary firing (reoxidation)) in an oxidizing atmosphere to convert crystal grain boundaries to insulators. The relative dielectric constant ∈r of SrTiO3 itself is about 200 and, therefore, is small. However, since the crystal grain boundaries have a capacitance, the apparent relative dielectric constant ∈rAPP can be increased by increasing the crystal grain size and reducing the number of crystal grain boundaries.
For example, in Patent Document 2, a SrTiO3 based grain boundary insulation type semiconductor ceramic element assembly having an average grain size of crystal grains of 10 μm or less and a maximum grain size of 20 μm or less is proposed. This is a semiconductor ceramic capacitor having a single-layered structure. In the case where the average grain size of crystal grains is 8 μm, a semiconductor ceramic element assembly having an apparent relative dielectric constant ∈rAPP of 9,000 can be obtained.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-302072
Patent Document 2: Japanese Patent No. 2689439
However, if reduction in thickness and multilayering of ceramic layers are pushed forward by using the dielectric ceramic described in Patent Document 1, there are problems in that the relative dielectric constant ∈r decreases, the temperature characteristic of the capacitance deteriorates, and short-circuit failures sharply increase.
Consequently, in the case where it is attempted to obtain a thin monolithic ceramic capacitor having a large capacity of, for example, 100 μF or more, the thickness of the dielectric ceramic layer is required to be about 1 μm per layer and the number of laminated layers is required to be about 700 layers to 1,000 layers, so that commercial application is difficult in this situation.
On the other hand, the SrTiO3 based grain boundary insulation type semiconductor ceramic described in Patent Document 2 has good frequency characteristic and temperature characteristic and a small dielectric loss tan δ. The electric field dependence of the apparent relative dielectric constant ∈rAPP is small and, furthermore, a varistor characteristic is provided, so that breakage of the element can be avoided even when a high voltage is applied. Consequently, an application to the field of capacitors is expected.
However, regarding this type of semiconductor ceramic, a large apparent relative dielectric constant ∈rAPP is obtained by increasing the grain sizes of crystal grains, as described above. Therefore, if the grain sizes of crystal grains decrease, the apparent relative dielectric constant ∈rAPP becomes small so as to cause deterioration of the dielectric characteristic. Consequently, there is a problem in that it is difficult to allow the facilitation of reduction in layer thickness and the improvement of dielectric characteristic to become mutually compatible.
Furthermore, in order to commercially apply the semiconductor ceramic to a monolithic ceramic capacitor, it is required to ensure a sufficient insulating property even when the layer thickness is reduced. However, regarding the monolithic semiconductor ceramic capacitor, the insulating property comparable to that of the monolithic ceramic capacitor is not ensured in practice in the present situation.